The present invention relates generally to a Doppler frequency angle measurement technique, to measure an uncooperative radar's AOA (Angle-Of-Arrival), for a single aircraft application, employing two or more antennas with a frequency measurement receiver which accomplishes accurate phase measurement and appropriate processing.
The conventional Instantaneous Frequency Measurement (IFM) receiver is a radio frequency receiver used primarily in electronic warfare (EW). Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple, calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length. Since the phase differences between the delayed and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video provides signals whose amplitudes are related to the phase delay. These video signals typically take the form sin .omega..tau. or cos .omega..tau., where .omega. is the angular frequency of the processed input signal and .tau. is the delay time. The sin .omega..tau./cos .omega..tau. are delivered to the encoding network which makes amplitude comparisons of the signals, determines the numerical value of .omega., and generates the digital frequency descriptive word.
An IFM receiver has many attractive features for EW applications, such as small size, light weight, wide instantaneous bandwidth, and fine frequency resolution.
In a digital RF receiver, the incident radiation is mixed with a local oscillator signal and down converted to an intermediate frequency (IF). This IF signal is discretely sampled and further processing is done using digital techniques. The frequency of the incident radiation may be determined by performing a discrete Fourier transform on the sampled signal.
United States patents by applicant James B. Y. Tsui, sole or et al, relating to frequency measurement receivers include (1) U.S. Pat. No. 4,663,516 issued Dec. 30, 1986 for an Instantaneous Frequency Measurement Receiver With Digital Processing; (2) U.S. Pat. No. 4,963,816 issued Oct. 16, 1990 for an Instantaneous Frequency Measurement (IFM) Receiver With Only Two Delay Lines; (3) U.S. Pat. No. 5,099,243 issued Mar. 24, 1992 for a Digital Frequency Measurement Receiver With Bandwidth Improvement Through Multiple Sampling of Complex Signals; and (4) U.S. Pat. No. 5,109,188 issued Apr. 28, 1992 for an Instantaneous Frequency Measurement Receiver With Bandwidth Improvement Through Phase Shifted Sampling of Real Signals. These patents are hereby incorporated by reference.
The following United States patents are of interest.
U.S. Pat. No. 4,825,213--Smrek PA0 U.S. Pat. No. 4,746,924--Lightfoot PA0 U.S. Pat. No. 3,812,493--Afendykiw et al. PA0 U.S. Pat. No. 3,735,400--Sletten et al.
The above patents relate to systems and techniques for the location and tracking of targets. In particular, the Smrek patent describes a radar system and technique for use in detecting and tracking targets. The radar system is operatively connected to simultaneously receive signals at three antenna apertures. The signals are phase shifted and compared to produce difference signals representative of a target s motion and location.
The Lightfoot patent is directed to apparatus and methods for locating a target aircraft from a receiver aircraft by utilizing emissions from non-cooperative illuminators. The receiver includes a pair of wing tip antennas for receiving non-reflected emissions from the illuminators to determine the range from the receiver aircraft to the illuminators. Calculation of the location of the target is accomplished based on range and time differentials between the receipt of the reflected signals at the antennas and the receipt of a corresponding direct signal. The determination of the bearing of the target and the illuminator relative to the receiver is accomplished by an amplitude comparison of the signals received at multiple ports of a multiple beamed array antenna.
The Afendykiw et al patent relates to a radar system for determining the position of a target. The system uses an interferometer antenna and cross-correlation techniques to measure the time delay in receiving a reflected signal from the target as compared to receiving a direct signal from the signal source. Additionally, a plurality of interferometer antennas and cross-correlation techniques are used to measure the relative phase difference between the signals received the antennas to determine the angle of arrival of the reflected signals.
The Sletten et al patent describes an airborne moving target radar system utilizing three radar antennas placed on an aircraft in a line coincident with the aircraft flight velocity vector. Radar echo signals received by these antennas are filtered into narrow band channels. Signals in the clutter frequency are canceled, while signals from moving targets are compared by a phase comparator. The system then operates as an interferometer to provide target bearing information.